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The effect of free ammonia on volatile fatty acid (VFA) accumulation and process instability was studied using a lab-scale anaerobic digester fed by two typical bio-wastes:fruit and vegetable waste (FVW) and food waste (FW) at 35℃ with an organic loading rate (OLR) of 3.0 kg VS/(m3·day).The inhibitory effects of free ammonia on methanogenesis were observed due to the low C/N ratio of each substrate (15.6 and 17.2,respectively).A high concentration of free ammonia inhibited methanogenesis resulting in the accumulation of VFAs and a low methane yield.In the inhibited state,acetate accumulated more quickly than propionate and was the main type of accumulated VFA.The co-accumulation of ammonia and VFAs led to an inhibited steady state and the ammonia was the main inhibitory substance that triggered the process perturbation.By statistical significance test and VFA fluctuation ratio analysis,the free ammonia inhibition threshold was identified as 45 mg/L.Moreover,propionate,iso-butyrate and valerate were determined to be the three most sensitive VFA parameters that were subject to ammonia inhibition.